Since the advent of the polymerase chain reaction (PCR), several variations to this nucleic acid amplification reaction have been devised. Additionally, several distinct nucleic acid amplification reactions have been introduced. For example, the ligase chain reaction (LCR) is an effective means for amplifying a nucleic acid sequence. Both PCR and LCR can be used to detect, for example, a pathogen in a test sample by amplifying a nucleic acid sequence unique to the particular pathogen (sometimes called a target sequence), then detecting the amplified nucleic acid sequences. The amplified nucleic acid sequences can be detected using techniques similar to those used in heterogeneous immunoassays.
A challenge facing the further development of amplification reactions includes detecting multiple target sequences in a test sample. Multiple target sequences can be detected to determine the presence of multiple pathogens that may be present in a test sample, or alternatively, multiple target sequences can be detected to quantify a target sequence present in a test sample. Unfortunately, methods for detecting multiple target sequences, for whatever purpose, is somewhat limited by the methods for detecting the signal generating groups that can be associated with the amplified sequences. In particular, in order to detect multiple target sequences, the sequences must be distinguished from one another. While such distinctions can be made by associating the sequences with different signal generating moieties, difficulties are presented when the signals from these moieties are detected. For example, when multiple fluorescent moieties are employed, each of the multiple moieties may have a distinct absorbtion and emission wavelength which can be employed to distinguish one sequence from another. But this detection scheme calls for a complex detection system that can excite and detect fluorophores at multiple wavelengths. Moreover, as the number of different fluorescent moieties to be detected increases, so does the complexity of the optical system employed to detect the moieties. Unfortunately, such systems are limited by the number of different sequences which can be detected because the complexity of the optical system increases in a cost prohibitive manner.
Alternatively, using multiple enzymatic signal generating moieties has been proposed to detect multiple target sequences, but such a detection scheme uses complex reagent systems to produce and inhibit signals generated by the enzymes. As a result, the predominant method for detecting multiple nucleic acid sequences is gel electrophoresis which distinguishes nucleic sequences based upon molecular weight. Gel electrophoresis, however, is a labor intensive, and therefore time consuming, method of detection which is not amenable to automation or standardization. Thus, there is a need for a nucleic acid detection system which is capable of detecting a plurality of target sequences in a practical manner.